[RFC] diff: add `diff.<driver>.process` for external hunk providers #28823

Job	Run time
config	9s
fuzz smoke test	1m 19s
win+Meson build	4m 0s
win build	5m 14s
static-analysis	6m 8s
osx-meson (macos-14)	47m 39s
osx-reftable (macos-14)	33m 11s
osx-clang (macos-14)	33m 8s
osx-gcc (macos-14)	32m 43s
documentation	7m 14s
sparse	3m 43s
rust-analysis	1m 36s
linux-reftable-leaks (ubuntu:rolling)	49m 12s
linux-sha256 (ubuntu:rolling)	25m 0s
linux32 (i386/ubuntu:20.04)	1h 6m 11s
linux-breaking-changes (ubuntu:rolling)	27m 44s
debian-11 (debian:11)	15m 12s
linux-meson (ubuntu:rolling)	23m 28s
fedora-breaking-changes-meson (fedora:latest)	19m 34s
linux-asan-ubsan (ubuntu:rolling)	1h 1m 9s
linux-TEST-vars (ubuntu:20.04)	23m 54s
linux-leaks (ubuntu:rolling)	58m 30s
almalinux-8 (almalinux:8)	21m 50s
linux-reftable (ubuntu:rolling)	16m 44s
linux-musl-meson (alpine:latest)	17m 58s
win+VS build	0s
win+VS test	0s
win+Meson test (4)	11m 42s
win+Meson test (5)	21m 38s
win+Meson test (1)	6m 47s
win+Meson test (7)	8m 1s
win+Meson test (0)	7m 0s
win+Meson test (2)	7m 58s
win+Meson test (6)	12m 24s
win+Meson test (3)	44m 45s
win+Meson test (8)	9m 58s
win+Meson test (9)	8m 10s
win test (7)	15m 45s
win test (2)	7m 24s
win test (8)	8m 0s
win test (6)	7m 44s
win test (3)	7m 52s
win test (9)	6m 58s
win test (1)	6m 57s
win test (0)	11m 11s
win test (5)	41m 48s
win test (4)	10m 28s
	14h 25m 0s

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